Pneumatic cylinders have a cylinder body comprised of a cylinder tube, a cylinder block and the like, a piston to be housed in a cylindrical chamber formed in the cylinder body so as to freely reciprocate therein, and a piston rod which protrudes outside of the cylinder body integrally with the piston. The reciprocating motion of the piston caused by supplying and exhausting a working fluid to/from both end surfaces of the piston is transmitted to the outside via the piston rod.
The pneumatic cylinders are divided, according to working forms, into single acting cylinders in which compressed air as the working fluid is supplied only to one of pressure chambers formed on both sides of the piston and movement to the opposite direction is made by means of a spring or an external force, and double acting cylinders in which a piston rod advances and retreats both by means of a compressed air. Further, a pneumatic cylinder in which a piston rod is provided on both sides of the piston is called as a double rod end cylinder, and a pneumatic cylinder in which a piston rod is provided to one side is called as a single rod cylinder.
Apparatuses using the pneumatic cylinder include a conveying apparatus or the like for conveying work pieces from steps to steps of a production line. For example, in the case where electronic parts such as semiconductor chips are inspected, a lot of electronic parts arranged on a tray or the like are mounted onto an inspection board by a conveying apparatus. Further, also in the case where the electronic parts are mounted onto a mounting board, the electronic parts are mounted onto the mounting board in a predetermined order by using the conveying apparatus.
Such a conveying apparatus is provided with a conveying head which is movable to a horizontal direction, and an up-down moving member which is movable to a vertical direction is attached to the conveying head. Further, a sticking tool is attached to a front end of the up-down moving member, the stuck electronic parts are ascended, moved horizontally and descended, so as to be conveyed to a predetermined position. A pneumatic cylinder is used as the up-down moving member. The sticking tool is attached to a front end of the piston rod and compressed air is supplied to the pneumatic cylinder so as to be controlled. As a result, the electronic parts can be ascended and descended.
Such a pneumatic cylinder requires a sealing performance which holds air pressure difference between the front and back of the axial direction at a sliding portion between the piston and piston rod, and a cylinder body. For this reason, a packing using an elastic material such as rubber or resin is fitted to an outer periphery of the piston and the piston rod, so that the outer periphery of the piston and piston rod are made to slide and contact with an inner peripheral surface of the cylinder body. In the case where the pneumatic cylinder is continuously used, however, a temperature rises so that the packing is easily damaged, and a sliding load at the time of the sliding of the piston is centered on the outer periphery of the packing so that sliding resistance becomes higher. Thus, there arises a problem that an operating cost in the case where the pneumatic cylinder is used in the conveying apparatus or the like increases.
For this reason, in recent years, instead of the packing, a slide bearing using a porous material is incorporated into the sliding portion of the cylinder body, and compressed air is always supplied from its outer periphery to the inner portion so as to form an air layer on the sliding portion (a gap between the piston or the piston rod and the slide bearing), so that the sliding resistance is reduced and the sealing performance is improved. Such an air bearing is developed.
A performance of the air layer formed by the air bearing is, however, remarkably influenced by a size of the gap of the sliding portion, and when a temperature is changed by continuous use or the like, the gap of the sliding portion easily increases or decreases greatly. For this reason, also the sliding performance and the sealing performance of the air bearing might increase or decrease greatly.
It is an object of the present invention to provide a pneumatic cylinder which is capable of maintaining the stable sliding performance and sealing performance regardless of the temperature change even if the air bearing is used.